RING-RESONATOR FOR LASERS WITH ANNULAR GAIN MEDIA

A ring resonator for lasers with annular gain media is presented. The resonator consists of two annular mirrors. While the radiation is reflected back and forth between the two annular mirrors, diffraction effects induce an additional azimuthal radiation flux. Output coupling is obtained through a decentered coupling aperture on the circumference of one of the two mirrors. The azimuthal radiation flux permits the extraction of optical power from the whole gain volume through the coupling aperture. The azimuthal radiation flux can revolve in two directions. The associated modes are degenerate, and random jumping between unidirectional and bidirectional operation is observed. Unidirectional operation has been stabilized but remains very sensitive to mirror alignment. High extraction efficiencies have been demonstrated experimentally with this resonator with a diffusion-cooled CO2 laser and 2 times diffraction-limited beams have been obtained. An empty resonator model that shows the effect of edge diffraction at the coupling aperture on the resonator modes is also given.